1. Field of the Invention
The present invention relates to a circuit for preventing destruction of a semiconductor laser element, where the circuit is provided with functions for controlling optical output of the semiconductor laser element. The present invention also relates to an optical disc device provided with the circuit.
2. Description of Related Art
Conventional optical disc devices for using laser light outputted from a semiconductor laser element installed on an optical pickup to record information on DVDs or other optical discs (optical-disc recording media) and play back the information recorded on the optical disc are widely used.
In such an optical disc device, the optical output of the semiconductor laser element is adjusted to a constant level by an automatic power control (APC) function. FIG. 3 is a configuration diagram that shows an example of a conventional optical disc device. As shown in FIG. 3, a conventional optical disc device 100 is provided with an optical pickup part 110, an LD driver 121 having APC functionality, an LD power source 122, an LD-driver power source 123, a switch circuit part 124, a control circuit part 130, a playback signal-processing circuit part 140, and an audio-visual output part 150. The optical pickup part 110 has a laser diode (LD) 111, a beam splitter 112, and a photodiode (PD) 113. The LD power source 122 supplies a light-emitting voltage VLD for causing the LD 111 to emit light. The LD-driver power source 123 drives the LD driver 121.
The laser light shone from the LD 111 is led by the beam splitter 112 to an optical disc 200 that is set in the optical disc device 100. Optical spots are thereby formed by the laser light on the optical disc 200. The laser light reflected by the optical disc 200 is led to the PD 113 by the beam splitter 112. The PD 113 performs a photoelectric conversion on the reflected laser light and generates an electrical signal containing information recorded on the optical disc 200 for playback. A video signal and an audio signal are generated by the playback signal-processing circuit part 140 on the basis of this electrical signal. The audio-visual output part 150 is used to implement a video display on the basis of the video signal and to implement audio output on the basis of the audio signal.
The electrical signal generated by the PD 113 is also inputted to the LD driver 121. The LD driver 121 adjusts the light-emitting electrical current provided to the LD 111 using feedback control on the basis of the inputted electrical signal, and to the extent possible limits fluctuations in the optical output of the LD 111.
However, when the light-emitting voltage VLD supplied to the LD driver 121 fluctuates faster than the response speed of this feedback control, the optical output control is unable to respond to the voltage fluctuations. In such instances, the optical output control of the LD driver 121 runs out of control, and the LD 111 is destroyed.
In cases where the optical disc device 100 is, e.g., a top-loading system (clam-shell type), the starting/stopping of light emission by the LD 111 must be associated with the opening and closing of the disc cover. When the disc cover is closed, a sensor switch is depressed, the switch circuit part 124 switches on the connection between the LD driver 121 and the LD power source 122, and the LD 111 can emit light. In this optical disc device 100, the disc cover may momentarily separate from the sensor switch when vibrations are applied during illumination of the LD 111. In such cases, the switch circuit part 124 momentarily switches off the connection between the LD driver 121 and the LD power source 122 in association with this momentary separation. The voltage supplied to the LD driver 121 is therefore momentarily lower than a light-emitting voltage threshold of the LD 111. When the LD 111 stops emitting light, the LD driver 121 sets the optical output of the LD 111 to an extremely high value. However, when the light-emitting voltage VLD is then immediately supplied to the LD driver 121, the optical output of the LD 111 is still set to an extremely high level, and therefore excessive electrical power is supplied to the LD 111, and the LD 111 is destroyed.
In response to this phenomenon, in Japanese Laid-Open Patent Application No. 2004-355697, when the driving current of the LD exceeds a permitted value, a current limiter stops the increase in driving current, and the magnitude of the driving current of the LD is limited so as to be less than or equal to a constant value, whereby destruction of the LD is prevented. However, even in Japanese Laid-Open Patent Application No. 2004-355697, when the driving current increases faster than the response speed of the current limiter, and the permissible value is exceeded, increases in the driving current cannot be stopped, and the LD is destroyed.